1. Field of the Invention
The present invention relates to a CO2 recovery system and a solid-particle removing method for use in the CO2 recovery system that can filter out solid particles contained in CO2-absorbing solution that is used for removing CO2 from exhaust gas and then remove the filtered-out solid particles.
2. Description of the Related Art
In recent years the greenhouse effect has been pointed out as one of causes of the global warming, and a countermeasure against it is urgently required internationally to protect global environment. CO2 emitted into the atmosphere has been considered the prime cause of the greenhouse effect. CO2 sources range various fields of human activities, including burning of fossil fuels, and demands to suppress their CO2 emission from these sources are on constant increase. Scientists have energetically studied means and methods for suppressing emission of CO2 from power generation facilities such as power generation stations which use an enormous amount of fossil fuels. One of the methods includes bringing combustion exhaust gas of boilers into contact with an amine-based CO2-absorbing solution. This method allows removal and recovery of CO2 from the combustion exhaust gas. Another method includes storing recovered CO2, i.e., not returning the recovered CO2 to the atmosphere.
Various methods are known to remove and recover CO2 from combustion exhaust gas using the CO2-absorbing solution. Japanese Patent Application Laid-Open No. H5-245339 discloses a method of contacting the combustion exhaust gas with the CO2-absorbing solution in an absorption tower, heating an absorbing solution having absorbed CO2 in a regeneration tower, and releasing CO2, regenerating the absorbing solution, and circulating the regenerated absorbing solution to the absorption tower again to be reused.
As shown in FIG. 7, in a conventional CO2 recovery system 1000, CO2-containing exhaust gas 1002 discharged from a factory 1001 is cooled with coolant water 1003 in a cooling tower 1004. The factory 1001 can be a boiler. The cooled CO2-containing exhaust gas 1002 is then conveyed to an absorption tower 1006 where it is brought into countercurrent contact with CO2-absorbing solution 1005. The CO2-absorbing solution 1005 can be an alkanolamine-based solution. CO2 in the CO2-containing exhaust gas 1002 is absorbed into the CO2-absorbing solution 1005, that is, CO2 is removed from the CO2-containing exhaust gas 1002. The CO2-absorbing solution 1005 containing CO2 (hereinafter, “rich solution 1007”) is conveyed to a regeneration tower 1008. The rich solution 1007 drips downward in the regeneration tower 1008. When the rich solution 1007 reaches a lower portion of the regeneration tower 1008, most of the CO2 absorbed in the rich solution 1007 is released, and the rich solution 1007 turns into lean solution 1009 capable of working as the CO2-absorbing solution 1005. The lean solution 1009 is heated with saturated steam 1030 in a regeneration heater 1014. Thereafter, the lean solution 1009 is returned to the absorption tower 1006 and it is reused as the CO2-absorbing solution 1005. The saturated steam 1030 after use is discharged from the regeneration heater 1014 as steam concentrated water 1031.
In the CO2 recovery system 1000, material such as a sulfur oxide (SOx) remain un-removed in a desulphurization step. Such residual material reacts with alkanolamine contained in the CO2-absorbing solution 1005 in the CO2-removing process thereby producing a thermostable salt. The thermostable salt mixes with the lean solution 1009, which creates various issues. The thermostable salt cannot be removed under normal conditions in a course of producing the lean solution 1009 from the rich solution 1007, so that the thermostable salt accumulates in the system while the lean solution 1009 circulates.
The CO2 recovery system 1000 includes a reclaimer 1040 to which the lean solution 1009 that is produced in the regeneration tower 1008 is supplied. The reclaimer 1040 heats the lean solution 1009 to produce a condensed depleted material such as a salt. The condensed depleted material is then removed.
More particularly, the lean solution 1009 passing through a lean-solution supply line 1022 is extracted through an extracting line 1041 that is stretched from the lean-solution supply line 1022 to the reclaimer 1040. The reclaimer 1040 receives saturated steam 1046 from a saturated-steam supply line 1045 into a saturated-steam supply pipe 1050 and heats the lean solution 1009 with the saturated steam 1046. The depleted material is removed from the extracted lean solution 1009 in the reclaimer 1040. The lean solution 1009 is heated in the reclaimer 1040 to, for example, 130° C. to 150° C., so that vaporized CO2-absorbing solution 1047 is obtained from the lean solution 1009. The vaporized CO2-absorbing solution 1047 is supplied to the lower portion of the regeneration tower 1008. A condensed waste-product accumulated on a bottom of the reclaimer 1040 is removed from the system, for example, the boiler, for example, by pumping with a pump.
In the conventional CO2 recovery system 1000, solid particles such as soot dust or fly ashes (coal ashes) remain un-removed by a desulfurization device (not shown) are removed in the absorption tower 1006. However, a part of the solid particles still remain un-removed in the lean solution 1009. The solid particles in the lean solution 1009 are filtered out by a filtering member, and the filtered-out solid particles are then removed. The filtering member after use is replaced with a new one and the old filtering member is discarded as a waste product. The filtering member can be, for example, a cartridge filter or a precoat filter.
If an amount of the solid particles in the lean solution 1009 is large, the filtering member needs to be replaced frequently, which brings heavy workload and produces a large amount of the waste product.
Assuming that the amount of the exhaust gas from the CO2 recovery system 1000 is, for example, about 1,000,000 Nm3/h and the amount of dust out of the exhaust gas is, for example, about 5.0 mg/Nm3, about 40% of the dusts is removed in the cooling tower 1004 and the desulfurization device (not shown) and about 60% (i.e., the remaining dust) is removed in the absorption tower 1006. In other words, the amount of dusts removed in the absorption tower 1006 is as much as 1,000,000 (Nm3/h)×5.0 (mg/Nm3)×0.6%=3.0 kg/h.
A maximum collectable dust-amount of a typical filter is about from 100 grams to 200 grams per bottle. It means that 15 to 30 filters are replaced every hour. Such frequent filter replacement is impossible in practical, and the conventional filtering cannot use in the large-capacity CO2 recovery system.
The lean solution 1009 contains the CO2-absorbing solution 1005 that contains amine. Therefore, a chemical oxygen demand (COD) of the lean solution 1009 is high, and the lean solution 1009 cannot be drained out as effluent.
There is a need for producing a large-capacity CO2 recovery system, such as the amount of exhaust gas is, for example, 1,000,000 Nm3/h and the amount of dust removed in the absorption tower is as much as, for example, 3.0 kg/h, that can remove without breaks a large amount of the solid particles from the lean solution 1009.
The present invention has been achieved to solve the above problems in the conventional technology and it is an object of the present invention to provide a CO2 recovery system capable of recovering a large amount of CO2 thereby producing dust to be removed in the absorption tower as much as, for example, 3.0 kg/h and a solid-particles removing method for use in the CO2 recovery system that can remove the solid particles from the lean solution.